Youbin Yu

To explore the effect of density on large yellow croaker (Larimichthys crocea) under intensive aquaculture conditions and determine the appropriate culturing density, this study investigate the effects of different stocking densities on the nutritional composition, stress, and immune levels of large yellow croaker. Through a long-term aquaculture experiment, conducted under flow-through conditions of intensive aquaculture, three initial density groups were set: a low density group [LD], 4.92 kg/m³; a medium density group [MD], 7.56 kg/m³; and a high density group [HD], 10.08 kg/m³, for a 150-day rearing trial. Large yellow croaker were fed to satiation twice daily (6:00, 17:00). At the end of this trial, the final densities were 10.38 ± 0.50, 14.41 ± 1.06, and 18.71 ± 0.99 kg/m³ in the LD, MD, and HD groups, respectively. The results showed that the growth performances were adversely influenced by a high stocking density. Levels of cortisol in serum, superoxide dismutase (SOD) and catalase (CAT) in liver, Na⁺-K⁺ ATPase and Na⁺-K⁺ ATPase gene in gills, and heat shock protein (HSP70/90) genes and glutathione S-transferase (GST) genes in the liver significantly increased under HD treatment. Results of immune response analyses showed that there was a clear decrease in immunoglobulin M (IgM), complement component 4 (C4), and lysozyme (LZM) in serum, lysozyme (LZM) genes, tumor necrosis factor-alpha (TNF-α) genes and interleukin-1β (IL-1β) genes in the head kidney of large yellow croakers reared in the HD group. An obvious increase in free amino acids and fatty acids in the muscle of large yellow croakers reared in HD group was also observed. Overall, this study showed that the optimal final culturing density of large yellow croaker under flow-through systems should be between 14.41 kg/m³ and 18.71 kg/m³ to improve aquaculture efficiency and product quality.

The noise analysis of a large-scale aquaculture vessel reveals that during its navigation, the primary equipment noise, particularly from the propeller, exerts a notable influence on the aquaculture environment for large yellow croaker. The free surface greatly impacts the noise performance of propellers, which is a significant factor affecting the fish’s habitat. This study adopts the numerical simulation method to analyze the hydrodynamic and acoustic characteristics of the E1619 propeller operating near the free surface. The open-water performance and noise calculations of the propeller are verified through experiments, and the effects of different immersion depths and advance coefficients on the propeller are explored. The results demonstrate that the free surface significantly affects the thrust, torque, and noise of the propeller, especially at shallow immersion depths and low advance coefficients. Surface wave pattern causes the instability and breakup of tip vortices, causing increased thrust and torque fluctuations, reduced efficiency, and significant overall sound pressure levels in the entire flow field. As immersion depth and advance coefficients increase, the interaction between tip vortices and the free surface weakens, wake vortex instability decreases, and noise levels gradually reduce. These analyses and conclusions can guide the design of next-generation propellers for aquaculture vessels to optimize performance near the free surface.

Ship aquaculture platforms are expected to become a meaningful way to expand offshore farming. The growth performance and nutritional composition of the large yellow croaker reared in an offshore ship aquaculture system and nearshore traditional cage system was evaluated in this study. The results showed that the aquaculture ship could effectively avoid the harsh environment such as typhoons and red tides. The test large yellow croaker adapted to the ship culture system in a short time. No serious stress events occurred during the whole rearing process. During the culture experimental period, the fish fed normally, and disease was controlled. The aquaculture ship has good environment conditions during breeding with a water temperature of 21.5–28.5 °C, salinity 20.7–31.8‰, pH 7.6–8.4, dissolved oxygen 7.2–12.8 mg/L, ammonia nitrogen < 0.08 mg/L, and the number of bacteria and vibrio in water were 1.2 ∗ 10³–1.6 ∗ 10³ CFU/mL and 1.2 ∗ 10²–1.8 ∗ 10² CFU/mL, respectively. The survival, weight gain rate, and monthly weight gain of the large yellow croaker in the ship were 99.02%, 41.48%, and 67.52 g, respectively, which were significantly higher than those of cage culture. The crude protein content of the large yellow croaker raised in the ship was significantly higher than that in the cage group, and the crude fat content was significantly lower than that in the cage group. These results indicated that the growth performance and nutritional composition of the large yellow croaker reared in offshore ship were better than those of the fish in the cage. These findings enhanced our understanding of an offshore ship aquaculture model of large yellow croaker.